Cousin Marriages May Be Taboo, but They're Not Genetic Disasters

In 2008, Discover had an article about cousin marriage:

In the western world, marriage between first cousins is labeled incest or inbreeding, and in the United States the practice is banned or restricted in 31 states. But a new essay argues that such laws are based on an outdated notion of the genetic risks involved in cousins marrying and reproducing.

[T]hose laws "seem ill-advised" and "should be repealed," a geneticist and medical historian write.... "Neither the scientific nor social assumptions that informed them are any longer defensible" [Scientific American].

First cousins share about an eighth of their genes, and are therefore more likely to receive two copies of some recessive gene that poses health problems. Scientists had assumed that the children of first cousins would therefore be more likely to be born with birth defects. But coauthor Hamish Spencer writes that the risk of congenital defects is about 2 per cent higher than average for babies born to first-cousin marriages – with the infant mortality about 4.4 per cent higher – which is on a par with the risk to babies born to women over 40. "Women over the age of 40 have a similar risk of having children with birth defects and no one is suggesting they should be prevented from reproducing," said Professor Spencer [The Independent].